In a field of an optical pickup apparatus, there has recently been an advance of a trend toward a shorter wavelength of a laser source used as a light source for reproducing information recorded on an optical disc, or for recording information on an optical disc, and for example, a laser source having a wavelength of 405 nm such as a violet semiconductor laser and a violet SHG laser wherein a wavelength of a red semiconductor laser is converted by using a second harmonic generation, have lately been put to practical use.
When these violet laser sources are used, information of 15-20 GB can be recorded in an optical disc having a diameter of 12 cm, in the case of using an objective lens with a numerical aperture (NA) identical to that of a digital versatile disc (hereinafter referred to as DVD for short), while, when NA of an objective lens is enhanced to 0.85, information of 23-27 GB can be recorded in an optical disc having a diameter of 12 cm. Hereafter, an optical disc and a magneto-optical disc using a violet laser source are called “a high density optical disc” generically, in the present specification.
As a high density optical disc, there are proposed two standards presently. One of them is Blu-ray disc (hereinafter referred to as BD for short) that uses an objective lens with NA 0.85 and has a 0.1 mm-thick protective layer, and the other is HD DVD (hereinafter referred to as HD for short) that uses an objective lens with NA 0.65-0.67 and has a 0.6 mm-thick protective layer. When considering possibility that high density optical discs of these two standards may circulate in the market in the future, a compatible optical pickup apparatus that can record and reproduce for both high density optical discs stated above as well as a conventional DVD is important, and among them, an optical pickup apparatus of a one-lens type wherein compatibility is attained by an objective lens is most ideal.
Especially, in recent years, development toward commercialization of high density optical discs has become brisk, and its application has been broadened not only to a player for exclusive use for reproducing but also to a recorder capable of recording and one housing therein a personal computer.
In a personal computer of a notebook type, as an application for housing therein a personal computer, a pickup optical system itself needs to be thin for achieving a thin type personal computer, and an optical element with a popular name of a slim type having a short focal length is indispensable for achieving a thin type.
For example, in the technology of three-wavelength compatibility (HD, DVD and CD (compact disc)) including HD, an effect of diffraction provided on the surface of an objective lens is frequently used for compatibility between HD and DVD, and a part of spherical aberration caused by a difference between a protective layer thickness of HD and that of CD is corrected, by making a laser light flux for CD to enter an objective lens as divergent light (for example, presumed by contents of description in Patent Document).
The reason for the light flux for CD to enter as divergent light is that NA of CD is smaller than that of HD or DVD and a wavelength of the light flux is long, and thereby, an amount of spherical aberration generated for CD is the smallest in these three types of optical discs.
(Patent Document 1) TOKKAI No. 2000-348376